1. Field of the Invention
The invention relates to a positioning method and a device, in particularly for a lithographic projection apparatus. More particular, the invention relates to a method of positioning an object such as a mask or a substrate to a required position on an object table in a lithographic projection apparatus comprising:                a radiation system for supplying a projection beam of radiation;        a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;        a second object table for holding a substrate; and        a projection system for projecting the patterned beam onto a target portion of the substrate.        
2. Background of the Related Art
The term “patterning means” or “mask” should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:    A mask held by said first object table. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the projection beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to a pattern on the mask. The first object table ensures that the mask can be held at a desired position in the incoming projection beam, and that it can be moved relative to the beam if so desired.    A programmable mirror array held by a structure, which is referred to as first object table. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference.    A programmable LCD array held by a structure, which is referred to as first object table. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once, such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
The projection system encompassing various types of projection system, including refractive optics, and/or reflective optics may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as, and including catadioptric systems, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference. The basic operating principle behind such a multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to an exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughout, which in turn improves the cost of ownership of the machine.
In a manufacturing process using a lithographic projection apparatus, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation sensitive material (resist). For this process it is necessary to position the substrate and the mask on respective object tables with a high accuracy, both with regard to each other and with regard to the tables.
If an object, such as a substrate 1 (see FIG. 2) is not positioned in a correct rotational position on an object table, such as a substrate table, 5 a position measurement error can occur during subsequent alignment of the substrate 1 to the mask. During alignment the substrate 1 is brought into the same rotational orientation as the mask, to which end it can be necessary to rotate the substrate table 5. An interferometer 9 used in a sensor system 7 can be sensitive to this rotation and give an error in the distance which is measured by using a laser beam 11 laterally directed to a side mirror on the table 5. Said error is a so-called beam-point error, which generally increases with increasing rotation of the table 5. The measurement error thus caused can give an error in the super-positioning of two concurrent images exposed on successive layers on the substrate 1. This error in the super-positioning of two concurrent images is generally called an overlay error.
Beam-point errors are caused by inconsistency in the orthogonality of mirror surfaces to interferometer beams. FIG. 3a shows an interferometer I that measures a distance L between the interferometer I and a mirror T, using a light beam pointed at the mirror T. As here depicted, the mirror T is rotated with dS with respect to the nominal incident beam, so that the angle between the beam of incidence and the beam of reflection is 2 dS. The total length of the interferometer beam is then B=L+L/(cos 2 dS). The distance L can accordingly be calculated from the total length B measured by the interferometer I and from the known rotation dS. Optimally, the interferometer beam is directed so as to be parallel to the X-direction in a given reference co-ordinate system. However, factors such as thermal instability and mechanical play can cause a transient deviation from this parallelism, which is referred to as the beam-point error. FIG. 3b shows a beam-point error dE at a rotation dS=0. The total length of the beam is B=L/(cos dE)+L/(cos dE). This formula shows that for small beam point errors dE the influence on the total beam lengths is small, however, if the mirror T is rotated with dS the influence of the beam-point error increases. FIG. 3c combines the error dE of FIG. 3b and the rotation dS of FIG. 3a. The total length of the beam is B=L/(cos dE)+L/cos (dE+2 dS). Differentiating this function and applying a small-angle approximation for dE and dS (dE typically being of the order of about 5 to 100 μrad), one obtains the expression dB/dE≈L*dS*dE. From this it is evident that, for relatively high values of dS, the sensitivity of B to beam-point errors dE increases.
The problem is further deteriorated in that an error in the rotational position of the mirror T (e.g. when mounted on the side of the substrate table 5 in FIG. 2) also has an influence on the measured distance. The influence of this error is twice as big as the beampoint error because, as shown in FIG. 3a, a mirror rotation has a double influence on the direction of the reflected beam. An error dEm in the mirror rotation has an influence on the measured total length B of the beam according to the expression dB/dEm≈2*L*dS*dEm. It is evident that, for relatively high values of dS, the sensitivity to errors dEm in the rotational position of the mirror T increases.
Both errors are shown as one-dimensional errors; however in reality these errors are two-dimensional, such that the error can be in the plane of FIG. 3a to 3c (as shown) and also in a direction perpendicular to said plane. Similar considerations apply to the case whereby the object 1 in FIG. 2 is a mask, and the object table 5 is a mask table.
Apart from the exposure problems caused by beam-point errors, further problems can arise if the object is wrongly positioned upon the respective object table. FIG. 4a shows a substrate 1 that is correctly positioned upon a vacuum generating surface 13. The substrate 1 covers the vacuum generating surface 13 in total, with only a small overlap between the border 15 of the vacuum generating surface 13 and the edge 2 of the substrate 1. Vacuum from the vacuum source 17 is applied to the vacuum generating surface 13 via the vacuum distribution means 19 and the vacuum chamber 21 to generate a vacuum force F on the substrate 1.
FIG. 4b shows a substrate 1 which is incorrectly placed upon a vacuum generating surface 13. The substrate 1 covers the vacuum generating surface 13 in total, but on one side too much overlap occurs between the edge 2 of the substrate 1 and the border 15 of the vacuum generating surface 13. On said one side, less vacuum force F can be applied to the edge 2; consequently, the substrate 1 can deform especially at the edge 2. The exposures on the substrate can fail because of image deformation on the non-planar edge.
FIG. 4c also shows a substrate 1 that is also incorrectly placed upon the vacuum-generating surface 13. The substrate 1 does not cover the vacuum generating surface 13 in total so that air A will enter the vacuum chamber 21 and the vacuum force F will be less than optimal. During exposure, the badly adhered substrate 1 can move over the vacuum generating surface 13, causing bad exposures to occur. If the substrate 1 gets totally loose, the substrate 1 can fall off the vacuum generating surface 13 and damage the surrounding apparatus. Same considerations apply to the case where the substrate 1 is held upon the object table 5 with electrostatic force. The latter may necessary when the invention is applied to an apparatus that is employed in vacuum.